WMM and U-APSD Power save mode

                                       For understanding WMM power save , its mandatory to understand QOS (WMM). Wireless networks have been widely adopted by all kinds of users. New applications which use video and multimedia streaming brought challenging quality of service (QoS) requirements like youtube and facebook.The growing demand for bandwidth has caused network congestion and slowdowns, but all users want multimedia distribution to work perfectly. These requirements have triggered the development of a QoS enhancement for the Wireless LAN.


 See the MAC header change in 802.11 Frame as below.

                 The CSMA/CA technique intended to provide fair and equal access to all devices. It is essentially a "listen-before-talk" mechanism. When networks become overloaded, the performance becomes uniformly poor for all users and all types of data. QoS modifies the access rules to provide a useful form of "controlled unfairness." Data that is identified as having a higher priority is given preferential access to the medium. It will therefore gain access at the expense of the lower priority traffic. Like Whenever we send data with FTP and other Voice call, Voice call will get get prrority compare to data.

EDCA  access is an extension of the legacy CSMA/CA DCF mechanism to include priorities. The contention window and backoff times are adjusted to change the probability of gaining medium access to favor higher priority classes. A total of eight user priority levels are available. Each priority is mapped to an access category, which corresponds to one of four transmit queues.

Each queue provides frames to an independent channel access function, each of which implements the EDCA contention algorithm. When frames are available in multiple transmit queues, contention for the medium occurs both internally and externally, based on the same coordination function, so that the internal scheduling resembles the external scheduling. Internal collisions are resolved by allowing the frame with higher priority to transmit, while the lower priority invokes a queue-specific backoff as if a collision had occurred. 

The parameters defining EDCA operation, such as the minimum idle delay before contention, and the minimum and maximum contention windows, are stored locally at the QSTA. These parameters will be different for each access category (queue) and can be dynamically updated by the QoS access point (QAP) for each access category through the EDCA parameter sets. 

These are sent from the QAP as part of the beacon, and in probe and re-association response frames. This adjustment allows the stations in the network to adjust to changing conditions, and gives the QAP the ability to manage overall QoS performance. 

Under EDCA, stations and access points use the same access mechanism and contend on an equal basis at a given priority. A station that wins an EDCA contention is granted a TXOP-the right to use the medium for a period of time. 

The duration of this TXOP is specified per access category, and is contained in the TXOP limit field of the access category (AC) parameter record in the EDCA parameter set. A QSTA can use a TXOP to transmit multiple frames within an access category. 

If the frame exchange sequence has been completed, and there is still time remaining in the TXOP, the QSTA can may extend the frame exchange sequence by transmitting another frame in the same access category. The QSTA must ensure that the transmitted frame and any necessary acknowledgement can fit into the time remaining in the TXOP

WMM power save mode (U-APSD)

• WMM power save mode is the enhancement to the  legacy power save mode.
• WMM Power Save was mainly designed for mobile and cordless phones that support VoIP.
• WMM Power Save promotes more efficient and flexible over-the-air transmission and power management by  enabling individual applications to control capacity and latency requirements
• VOIP applications are extremly sensitive to the delays.
• Incrased Latency is the side effect of Power save mode and WMM power save will address this.
• The application-based approach used in WMM Power Save enables individual applications to decide how often the  client needs to communicate with the access point and how long it can remain in a dozing state.
• In legacy power save mode it is based on listen interval irrespective of  active applications.
• Applications that do not initiate power save can still coexist with WMM Power Save enabled applications on the same device.
• In this case, data from the other applications will be delivered with legacy power save, while WMM Power Save  applications will still enjoy its additional functionality as long as the access point also supports WMM Power Save.

            The core technology used by WMM and WMM Power Save depend on enhancements to the 802.11 Media Access Control (MAC) layer.

WMM categorized Wi-Fi traffic in to four diffrent access categories

1.  Voice
2.  Video
3.  Best Effort
4.  Background

 For example, in a Wi-Fi network with WMM, voice receives priority over all other types of traffic, thus improving the performance of voice applications.

 WMM Power Save is based on Unscheduled Automatic Power Save Delivery (U-APSD)

 WMM Power Save improves the efficiency of legacy power save by increasing the amount of time the client is allowed to doze and by decreasing the number of frames that a client needs to send and receive, in order to download the same number of frames buffered by the access point as before. It consists of a signaling mechanism added to WMM that enables the access point to buffer data frames and send them to the client upon its request.

 Power save behavior is negotiated during the association of a client with an access point. WMM Power Save or legacy power save is set for each WMM AC (voice, video, best effort, background) transmit queue separately1. For each AC queue, the access point will transmit all the data using either WMM Power Save or legacy power save using the WMM QoS mechanism.

While clients using legacy power save need to wait for the beacon frame to initiate a data download, WMM Power Save clients can initiate the download at any time, thus allowing more frequent data transmission for applications that require them.

There are two ways in which the access point may send the buffered data frames to the client. If the data belongs to a legacy power-save queue, transmission follows legacy power save. If the data belongs to a WMM Power Save queue, data frames are downloaded according to a trigger-and-delivery mechanism.

The client sends a trigger frame on any of the ACs using WMM Power Save to indicate that it is awake and ready to download any data frame that the access point may have buffered. Unlike with legacy power save, the trigger frame can be any data frame, thus eliminating the need for a separate PS-poll frame which contains only signaling data.

After the client has sent a trigger frame, the access point acknowledges it is ready to send the data. Data frames are sent during an EDCA Transmit Opportunity (TXOP) burst, with each data frame interleaved with an acknowledgement frame from the client. On the last data frame, the access point indicates that no more data frames are available and the client can revert to its dozing state.

U-APSD steps :

1. The procedures apply to unicast QoS-Data and QoS-Null frames that are to be delivered to a WMM STA when the STA is in PS-mode. U-APSD shall only be used to deliver unicast frames to a WMM STA. Broadcast/multicast frame delivery shall follow the legacy frame delivery rules .

2. The WMM power-save procedures are based on the legacy procedures, but an option for unscheduled automatic power-save delivery (U-APSD) is added. WMM APs capable of supporting U-APSD shall signal this capability through the use of the U-APSD subfield (b7) in the QoS Info Field in Beacon, Probe Response and (Re)Association Response management frames.

3. In order to configure a WMM AP to deliver frames, the WMM STA designates one or more of its

ACs to be delivery-enabled ACs and one or more of its AC to be trigger-enabled ACs. A WMM

STA may configure a WMM AP to use U-APSD using two methods.

4. First, a WMM STA may set individual U-APSD Flag bits (b3~b0) in the QoS Info field of the WMM Information element carried in (re) association request frames (see §2.2.1). When a U-APSD Flag bit is set to 1, it indicates that the corresponding AC is both a delivery-enabled AC and trigger-enabled AC. When a U-APSD Flag bit is set to 0, it indicates that the corresponding AC is neither a deliver-enabled AC nor a trigger-enabled AC. When all four U-APSD Flag subfields are set to 1 in the most recent (re) association request frames, all the ACs associated with the WMM STA are trigger-enabled ACs and delivery-enabled ACs upon successful (re) association. When all four U-APSD Flag subfields are set to 0 in (re) association request frames,the ACs associated with the WMM STA are neither trigger-enabled ACs nor delivery-enabled ACs upon successful (re) association.

5. Alternatively, a WMM STA may request one or more AC as a trigger-enabled AC and one or more AC as delivery-enabled ACs by sending an ADDTS request per AC to the WMM AP with the PSB subfield (b10) in the TS Info field in the TSPEC element. In an ADDTS Response WMM AP must preserve the setting of the PSB subfield from the ADDTS Request. Requests to designate an AC as a delivery-enabled AC or trigger-enabled AC are admitted when the Status Code is equal to 0 in an ADDTS response. A WMM STA may request an AC to be a triggerenabled AC with a TSPEC with the PSB subfield set to 1 in the uplink direction. A WMM STA may request an AC to be a delivery-enabled AC with a TSPEC with the PSB subfield set to 1 in the downlink direction. A bi-directional TSPEC with the PSB subfield set to 1, makes an AC both a trigger-enabled AC and delivery-enabled AC. A bi-directional TSPEC with the PSB subfield set to 0, makes that AC neither a trigger-enabled AC nor a delivery-enabled AC.

6. APSD settings in an admitted TSPEC take precedence over the static U-APSD settings carried in the WMM Information element in the most recent (re) association request. 

Below is U-APSD operation with MoreData=1 

7. In other words, an admitted TSPEC overwrites any previous UAPSD setting of an AC. An acknowledged DELTS for a bi-directional TS or a sole unidirectional TS for an AC reverts that AC to the static U-APSD settings carried in the WMM Information element in the most recent (re) association request. 

8. If there are two admitted unidirectional TSs in an AC, an acknowledged DELTS for one of the TSs results in a U-APSD setting for the AC per the PSB bit from the TSPEC of the remaining TS.

9.  WMM STAs use the Power Management field (b12) in the frame control field  of a frame

to indicate whether it is in active or power-save mode. As U-APSD is a mechanism for the

delivery of downlink frames to powersaving stations, the uplink frames sent by a WMM STA

using U-APSD shall have the Power Management bit in the frame control field set to 1 for

buffering to take place at the WMM AP. WMM STAs may use U-APSD to have some or all

frames of delivery-enabled ACs delivered during Unscheduled Service Periods (USPs). A WMM

STA chooses legacy versus U-APSD behavior on a per-AC basis.

10. If, for a particular WMM STA, an AC is not a delivery-enabled AC, then all downlink frames

destined to that WMM STA that map to that AC are buffered and delivered using the procedures

described in [1]. The buffer used to hold these frames will be referred to as the legacy PS buffer.

The WMM AP uses the TIM and the More Data bit (b13) carried in Frame Control Field to

indicate the status of the legacy PS buffer .

11 Transmission of a Trigger Frame is not implicitly allowed by admission of a downlink TS. If the

Trigger Frame maps to an AC that has ACM=1, then the WMM STA must establish a suitable

uplink TS before sending Trigger Frames.

12 The WMM STA must remain awake as long as an USP is still in progress.

• To ensure backward compatibility, the beacon frame contains TIM information for WMM Power Save frames only if all transmit queues are trigger-and-delivery enabled. If one or more transmit queues uses legacy power save, the beacon frame only contains legacy power-save TIM information.
• VoIP application using WMM Power Save may save anywhere from 15 to 40% while keeping the impact on latency low.
• Several changes over legacy power save make these improvements possible:
•  The client can request a data download without having to wait for a beacon frame. This reduces latency for applications like VoIP that require low latencies and enables more efficient dozing periods when the client does not need to receive or transmit data
• All downlink data frames are sent together in a fast sequence, thus reducing the number of frames required to receive the same amount of data.
• The trigger frame in WMM Power Save is effectively a data frame, while the legacy PS-poll frame only includes signalling information. This effectively further reduces the number of frames sent by the client and it is particularly advantageous in applications like VoIP that need to send data frames and poll the access point very frequently.
• Applications specify the power-save behaviour, thus increasing the flexibility in setting dozing periods and in sending trigger frames. As a result, applications like VoIP will poll the access point frequently during voice calls, while a data application may have longer dozing periods because it can better tolerate longer latencies
• WMM Power Save can coexist with legacy power save. A WMM Power Save client will still work within a legacy network and run applications that do not support WMM Power Save. As a result, no upgrade is needed to accommodate WMM Power Save devices in existing networks, if WMM Power Save functionality is not required.
• However to take advantage of the benefits of WMM Power Save, both the client and the access point need to be Wi-Fi CERTIFIED for WMM Power Save. This enables the client and the access point to negotiate power-save behaviour upon association. The presence of other clients in the network that are not Wi-Fi CERTIFIED for WMM Power Save does not affect the use of WMM Power Save for those devices that support it